How to Test a Power Supply (Paperclip Test and Multimeter Steps)

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A power supply rarely announces that it is failing. Instead it shows up as random shutdowns during a game, a build that does nothing when you press the power button, or a machine that reboots only when the graphics card ramps up. Because the PSU sits underneath every other component, a weak one gets blamed for problems it did not cause and walks free from problems it did. Testing it directly is the fastest way to stop guessing and start narrowing the fault down to one part.

There are three practical tests you can run at home, and they answer different questions. A jumper test tells you whether the unit will switch on at all. A multimeter tells you whether each rail is delivering voltage inside the range the ATX standard allows. A test inside a running system tells you how the unit behaves under real load, which is where marginal units usually reveal themselves. This guide walks through all three in order, from safest to most revealing.

Before you test: safety and preparation

The capacitors inside a power supply can hold a charge long after the wall plug comes out, which is why every manufacturer’s manual tells you not to open the metal shell. Every test below is done from the outside, using only the connectors the unit already exposes.

1. Shut the PC down fully, flip the switch on the back of the PSU to the O position, then unplug the mains cable.
2. Press and hold the case power button for about ten seconds to drain residual charge from the motherboard.
3. Wait a few minutes before handling anything. Do not remove the four screws holding the PSU shell together.
4. Work on a hard, non-carpeted surface and touch a bare metal part of the case to equalise static before touching components.
5. Disconnect the 24-pin motherboard connector, the CPU (EPS) connector and every PCIe power cable, so the PSU is isolated from the rest of the build.
6. If the interior is packed with dust, take the opportunity to clean the dust out of your PC while everything is already unplugged.

How to run the paperclip test (jumper test)

An ATX power supply stays asleep until the motherboard pulls the PS_ON# line to ground. The jumper test does that job manually. It proves the unit powers up; it does not prove the voltages are correct.

1. With the PSU unplugged, lay the 24-pin connector on the bench with the clip facing you and the wires pointing away.
2. Find the single green wire. On a standard ATX pinout that is PS_ON#, and every black wire next to it is ground.
3. Straighten a paperclip and bend it into a U shape, or use a purpose-made jumper. Push one end fully into the green pin socket and the other into an adjacent black pin socket.
4. Attach a small load so the unit is not spinning up completely unloaded — an old case fan or a spare mechanical drive on a Molex or SATA connector is enough.
5. Plug the mains cable back in and flip the PSU switch to I. The internal fan should spin, though some modern units with a semi-passive mode stay still until they warm up, so check the manual before calling it a failure.
6. Switch off, unplug, and remove the paperclip before reconnecting anything.

A unit that refuses to spin at all, clicks repeatedly, or trips the breaker has failed this test conclusively. A unit that spins has only earned the right to move on to the next one.

How to test the voltages with a multimeter

This is the test that produces real numbers. Set a digital multimeter to DC voltage, 20 V range. Keep the jumper in place from the previous section so the PSU stays on, and keep the small load attached.

1. Touch the black probe to any black (ground) pin on the 24-pin connector and leave it there.
2. Touch the red probe to a yellow pin. That is the +12 V rail, the one that feeds the CPU and graphics card.
3. Move the red probe to a red pin for +5 V, then an orange pin for +3.3 V.
4. Check the purple pin, which carries +5 V standby. This one is live whenever the PSU is plugged in, even with the system off.
5. Check the grey pin, the Power OK signal. It should read close to +5 V once the unit is running.
6. Write each reading down rather than trusting your memory — you will want to compare them later if the fault returns.

Intel’s ATX power supply design guide, which the major manufacturers build to, allows a tolerance of five percent on the +12 V, +5 V and +3.3 V rails. In practice that means roughly 11.4–12.6 V, 4.75–5.25 V and 3.14–3.47 V. Anything outside those bands, or a reading that wanders while you watch it, points at a unit that should be replaced rather than nursed along. If you are already shopping, our roundup of modular power supplies worth building around is a good place to start, and it helps to know how to choose the right power supply wattage before you pick a model.

How to test a power supply inside a working PC

Bench tests run the PSU at almost no load, and plenty of failing units pass them. If the system boots but misbehaves, you need to watch the rails while the hardware actually pulls current.

1. Reconnect everything, boot into the operating system, and open a hardware monitoring utility that reports the +12 V, +5 V and +3.3 V rails.
2. Note the idle readings, then run a demanding game or a stress test that loads the CPU and GPU together for fifteen to twenty minutes.
3. Watch for rails that sag below the tolerance bands above as load rises, or that spike when load drops away suddenly.
4. Cross-check against a second source if you can. Motherboard sensors are convenient but not precise, so back a suspicious reading up with the multimeter on a spare Molex connector while the system runs.
5. If the machine shuts down mid-test with no blue screen, no error and no warning, treat the PSU as the prime suspect.

Troubleshooting: what the results actually mean

The tests narrow the fault down, but the readings still need interpreting.

1. Nothing at all, no fan, no standby voltage. Try a different wall socket and a known-good kettle lead first. If the purple standby pin reads zero with the mains connected, the unit is dead.
2. Fan spins, voltages in range, PC still will not post. The PSU is probably fine. Move on to the motherboard, RAM seating and the front-panel power switch header.
3. Voltages fine at idle, sagging under load. Classic sign of an ageing or undersized unit. Confirm the build is not simply over its budget before replacing.
4. +12 V drops only when the GPU spikes. Check that each PCIe connector runs on its own cable, and review your graphics card installation for a partially seated power connector.
5. Readings jump around erratically. Make sure your probe has solid contact in the pin socket. If the contact is good and the number still dances, the ripple filtering is likely on its way out.

Common mistakes to avoid

1. Opening the PSU shell to look inside. There is nothing user-serviceable in there and the capacitors are genuinely dangerous.
2. Testing with the 24-pin still attached to the motherboard, which risks damaging the board and gives meaningless readings.
3. Running the jumper test with no load at all — some protection circuits shut the unit down and you misread that as a failure.
4. Putting the paperclip into the wrong pins. Confirm the green wire before you insert anything, and never bridge two coloured wires.
5. Declaring the unit healthy after a bench test alone. If the symptom only appears under load, the bench test cannot see it.
6. Replacing a PSU with a unit chosen purely on wattage. Efficiency rating, rail design and connector count matter too, which is why comparing 850W units built for gaming PCs is worth the time.

Frequently asked questions